Previous studies used transient visual cues to indicate the need for stopping prepotent responses. The latency of countermanding signals for humans was estimated at around 100 ms (Logan, 1994), mediated by peripheral motor area in the brain (DeJong et al, 1994). The current studies investigate the latency and underlying mechanism of generating such signals. We asked human subjects to stop or reverse their saccade direction upon detection of altered text patterns during reading. These tasks are embedded in the process of reading, with the occurrence of certain text patterns indicating the need for manipulating gaze directions. These text changes are made during saccade using gaze-contingent display changes so that no transient visual events are involved. The prepotent responses are defined by the nature of reading tasks. We use hazard functions of saccade initiation during fixation to indicate the onset of inhibitory signals associated with the countermanding and reversed saccades.

The results show that the use of non-transient visual cues results in two separate neural events: an early inhibition associated with the occurrence of altered text patterns that is very automated (latency > 175ms), and a late control of gaze direction associated with task instructions, which is effortful and requires cognitive resources (latency > 275ms). Thus, although agreeing with the concept of horserace models proposed by Logan and colleagues, we think that their estimation reflects the results of implementing transient visual cues, which does not underlie the process of initiating the intended stop-signal and therefore underestimates its latency.

Our results suggest that the response time of regulating gaze direction, using visual cues, reflects the latency of a mixture of two distinctive neural signals, with the early signal being mediated by the frontal eye field, and the late and effortful signal being mediated by the executive working memory.